In radio communications systems, the transmitted signal travels from transmitter to receiver over a channel consisting of a number of different paths, known as multipaths. The transmitted signal component travelling along a particular path experiences absorption, reflection or scattering by any objects located along that path This causes the signal components arriving at the receiver to have different amplitudes, phases and delays so that they interred with one another. This interference is known as multipath fading, and results in Intersymbol Interference (ISI) at baseband. ISI describes the spreading out of the data symbols so that components of past and future symbols are superimposed on the current symbol, thus increasing the difficulty of symbol detection and causing symbol errors.
An equalizer is often used at the receiver to remove ISI caused by the channel, by effectively applying the inverse of the channel impulse response to the received data symbols. Such an operation removes the effects of the channel from the received data. IN order to operate correctly, the equalizer requires a known preamble (or training sequence) to be transmitted together with each burst (or packet) of data symbols. The channel characteristics are then calculated from the received training data and used to adapt the equalizer to remove the channel effects from the subsequent burst or packet of data symbols. A decision feedback equalizer (DFE) is an example of a type of equalizer commonly used in radio communication systems since it is able to deal with channels having spectral nulls.
The construction of a decision feedback equalizer is shown in our International Patent Application No. PCT/GB99/01801, the necessary elements of which are incorporated herein by reference and to which the reader is referred for. further details.
Equalization while reducing ISI, also has a number of disadvantages. More particularly, equalization using a training sequence or preamble introduces an overhead in data rate, since the preamble uses up symbols which could othcrwise he been used to carry data. Equalization also introduces an overbead in packet delay, since the equalizer requires a certain period of time to train itself to the channel with the help of the preamble. A separate equalizer is often required at both terminals (base station and subscriber terminal), which means that the equalization overhead is present on both the downlink (base station, to subscriber terminal transmission) and on the uplink (subscriber terminal to base station transmission). In a point-multipoint system with many subscriber terminals accessing a single base station and using a Time Division Multiple Access TDMA) channel access scheme for the uplink, the equalizer at the base station will be required to re-train for each burst received form a different subscriber terminal. This means that delays for re-training will be frequent, and will reduce the overall data rate.